1. Field of the Invention
The invention relates to detection and monitoring devices and is directed more particularly to an assembly for detecting and monitoring the presence of scour in underwater beds, such as river beds, navigational channels, and the like.
2. Description of the Prior Art
Scour is a severe problem that results in millions of dollars of damage to infrastructure and substantial loss of life annually. Scour occurs during times of high tides, hurricanes, rapid river flow, and icing conditions, when sediment, including rocks, gravel, sand, and silt, are transported by currents, undermining bridge and pier foundations, submarine utility cables, and pipelines, and filling in navigational channels. Scour is dynamic; ablation and deposition can occur during the same high energy hydrodynamic event. The net effect of scour has not heretofore been easily predicted, nor readily monitored, in realtime.
Bridge scour monitoring technologies are generally known. In U.S. Pat. No. 5,784,338, issued Jul. 21, 1998 to Norbert E. Yankielun et al, an instrument called a xe2x80x9ctime domain reflectomerxe2x80x9d (TDR) is directly connected to a parallel transmission line consisting of a pair of robust, specially fabricated non-corroding rods or wires (hereinafter xe2x80x9cleadsxe2x80x9d). The principle of TDR is generally known, described in the technical literature, and applied to numerous measurement and testing applications. The technique was applied to scour detection and monitoring in the aforesaid ""338 patent, which is incorporated herein by reference. TDR operates by generating an electromagnetic pulse, or a fast rise time step, and coupling it to a transmission line. The pulse travels down the transmission line at a fixed and calculable velocity. The pulse propagates down the transmission line until the end of the line is reached, and is then reflected back toward the source. The time in seconds that it takes for the pulse to propagate down and back the length of the transmission line is called the xe2x80x9cround trip travel timexe2x80x9d and is calculated as described in the ""338 patent.
For a two wire parallel transmission line, changes in the dielectric media in the immediate surrounding volume cause a change in the round trip travel time. Further, at any boundary condition along the transmission line (e.g., air/water and water/sediment), a dielectric discontinuity exists. As a pulse traveling down the transmission line from the TDR source encounters these boundary conditions, a portion of pulse energy is reflected back to the source from the boundary. A portion of the pulse energy continues to propagate through the boundary until another boundary, or the end of the cable, causes all or part of the remaining pulse energy to return along the transmission line toward the source. Measuring the time of flight of the two reflected pulses, and knowing the dielectric medium through which the pulse is traveling, permits calculation of the physical distance from the TDR source to the dielectric interface boundary, or boundaries, encountered.
Freshwater has a relatively high dielectric constant and dry sedimentary materials (e.g.: soil, gravel and stone) have a relatively low dielectric constant. Wet sediment has a dielectric constant that is a mixture of the constants of water and dry soil. The dielectric constant of this mixture will vary, depending upon the local sedimentary material constituency. However, in all cases of bulk dielectric, the bulk index of refraction of the mixture will be less than that of liquid water alone and significantly greater than that of the dry sedimentary materials. Some sediment materials, particularly clay-based sediments, can be extremely xe2x80x9clossyxe2x80x9d. This lossy behavior of the soil is exhibited by a severe attenuation of an electromagnetic pulse as it propagates along a transmission line surrounded by such materials. The pulse, when launched from a TDR, dissipates as it travels along the transmission line. In lossy consolidated soils, such as clay, the electromagnetic signal is greatly attenuated as it propagates along the imbedded transmission line leads. Levels of signal attenuation can be as much as 10""s of decibels per meter. This results in little or no reflected signal returned to the instrument over the length of the leads buried in the lossy media. If the sensor source is imbedded in lossy media, along with a portion of the sensor leads, the media can absorb all, or nearly all of the pulse energy, such that little or no reflected signal is returned. If a pulse is propagating along a transmission line imbedded in a non- or minimally-lossy material and a boundary with some extremely lossy material is encountered, a reflection will occur at the interface boundary, similarly to that that would occur for a boundary between two non-lossy materials. The magnitude of the reflection will be proportional to the reflection coefficient of the two materials at the interface.
When one or both of the reflected signals from the interface and the distal ends of the leads are severely weakened by attenuation, the TDR electronics can experience difficulty in discerning one from the other and in providing consistently accurate readings. As the loss factor of the surrounding material increases, the pulse propagating along an electronic TDR system is greatly attenuated. In lossy environments, such as brackish or saline water, the pulse often is attenuated to levels below what can be procured or recognized by the TDR system.
Accordingly, there is a need for a reflectometry system which provides distinct and useful signals reflected from a boundary layer on a continuing basis so as to provide reliable monitoring of scour conditions in real-time, even in lossy environments.
An object of the invention is, then, to provide a reflectometry system for operation in lossy environments and, which provides readily discernible signals reflected from a boundary layer and a distal end of a sensor, which signals are sufficiently robust for use in determining the position of the boundary layer on a real-time basis and thereby permit continuous monitoring of the boundary layer position.
With the above and other objects in view, as will hereinafter appear, a feature of the invention is the provision of a scour sensor assembly comprising an optical TDR system including a fiber optic cable for disposition in a water/sediment interface such that a first portion of a length of the cable is disposed in sediment below the interface and a second portion of the length of the cable is disposed in water above the interface. A light pulse assembly is fixed at a first end of the cable and adapted to send periodic light pulses through the cable toward a second end of the cable and to receive pulses reflected from irregularities in surface walls of the cable. The light pulse assembly is adapted to compute travel times of the reflected pulse and to determine therefrom depth of the interface on a continuing basis.
The above and other features of the invention, including various novel details of construction and combinations of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular assembly embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.